We have used synchrotron radiation for high resolution structural studies of plant viruses. Diffraction data were collected from crystals of Satellite Panicum Mosaic Virus (SPMV) leading to a 1.9 E resolution structure of the T=1 virus. Diffraction data from crystals of Turnip Yellow Mosaic Virus (TYMV), a T=3 virus with a very large unit cell, were also collected to 3.5 E resolution. These data, which are unattainable using conventional X-ray generators, are being used in the structural determination of TYMV. Plant virus structural studies are important in understanding virus structure, but also provide a convenient way to study the ways in which X-ray data collection and data processing of large unit cells can be done successfully.